Most aluminum is produced by the Hall-Heroult process which involves the electrolysis of alumina in a molten cryolite bath using carbon electrodes. The carbon anodes are consumed by the anodic oxidation process with the formation of CO.sub.2 /CO and their life-time is very short, typically about 2-3 weeks for the pre-baked type of anode. They also add impurities to the bath. The cathodes, which are also made of carbon but have a longer life-time of more than two years, are covered with a cathodic layer of molten aluminum which has to be maintained very thick in order not to expose the carbon to the bath because the carbon is not wettable by molten aluminum. This high inventory of aluminum in the cell leads to the drawback that the electro-magnetic forces produce waves and ripples in the molten aluminum which necessitates a large interelectrode gap and a corresponding high cell voltage.
Many materials and design expedients have been suggested and tried with a view to improving the performance of electrolysis, but so far the results have not been successful. In particular, there have been numerous suggestions for aluminum-wettable cathode materials such as the refractory borides, but these materials are expensive, difficult to manufacture, and difficult to fix as a cell lining material or to coat them on less expensive substrates. Various composite materials have also been suggested for this purpose (see for example U.S. Pat. Nos. 2,480,475; 3,328,280; 3,408,312, 3,459 515 and 3,661,436) but none of these materials has proven to be acceptable.